1. Field of the Invention
The present invention relates to a novel structure and method of producing a fiber reinforced composite article in which the surface of the article will generate a visible indication thereon upon impact of the surface by a foreign object.
2. State of the Art
The advantages of making high strength, relatively lightweight articles from fiber reinforced composites is well known in the prior art. Generally, the articles made of composite fiber reinforced materials are high in strength, and yet have a relatively low weight. Such articles as high strength tanks, vessels and structural members including drive shafts, aircraft structural members and other items are advantageously made of composite fiber reinforced materials.
Various procedures and apparatus have been developed for producing articles with multiple layers of filamentary material. Methods of fabricating high strength articles include winding of reinforcing filaments on a mandrel, wet lay up of fiber fabric, use of what is known as "pre-preg" fibers or fabric which have been impregnated with a resin matrix some time prior to use, etc. All these methods and materials are referred to as "fiber reinforced".
When items made of reinforced composites, and especially of high strength carbon or graphite fibers, are impacted at low energies and velocities, such as from handling bumps incurred during shipment, storage and assembly of the items as well as inadvertent impacts resulting from dropping of tools, or other articles on the items, little, if any, damage is visible on the impacted surfaces. However, shock waves traveling through the laminate structure produce significant discontinuities or defects within the thickness of the composite structure. Such damage reduces the strength of the composite article, and, unfortunately, the resulting damage is very difficult to discover when using conventional quality control methods.
Heretofore, there has been no inexpensive, reliable means for ascertaining possible damage due to low energy, low velocity impact on the composite structure without resorting to costly nondestructive testing of the entire article. Higher energy impacts can be discerned because of the surface damage and deformation which occurs. But, because of the lack of any visual indication of a low energy, low velocity impact, resort must be made to costly nondestructive testing of each and every item to determine if serious damage to the subsurface of the article may have occurred due to such low energy impacts. Because of the difficulty in determining possible damage from unintentional impacts which themselves may not be recognized as being harmful, it has been the practice to overdesign the system to offset any possible damage from such impacts. This partially alleviates the high costs required if all items were individually subjected to nondestructive testing, but the overdesign is itself costly, and there is still left a question as to the integrity of items which are not individually subjected to nondestructive testing.
A principal objective of the present invention is to provide a fiber reinforced composite structure in which damage caused by low energy, low velocity impacts of the surface of the structure can be readily detected.
It is also an objective of the invention to provide a process of fabricating articles of carbon, graphite, glass or like fiber reinforced composite material in which the article has a high strength to weight ratio and in which visual inspection of an article which has been subjected to a low energy, low velocity impact of the surface will reveal the damage.